President Donald Trump and Republican leaders unveiled a nine-page framework to rewrite the nation’s tax code this week to rave reviews from within their party. But now the hard part starts — with the tax-writing committees in the House and Senate tasked with settling some of the most divisive issues.

Questions from where to set the top income tax rate to whether the package should be paid for could easily fracture the GOP. With 52 senators in their ranks and little hope of Democratic support, Republicans can’t afford to lose more than two members to get a bill passed. They also can’t afford another legislative loss following their failure to repeal the 2010 Affordable Care Act.

“There’s no question that there’s certainly comfort in margins, and we don’t have margins for error. And so each individual senator is very empowered when it comes to a big issue like this,” No. 3 GOP Senator John Thune of South Dakota said, referring to the tax overhaul. “As we’ve seen now a couple different times, it’s very easy to take a big bill like this down.”

The fate of Trump’s promise of a historic tax revamp could be determined by six key Republican senators: Bob Corker, John McCain, Rand Paul, Pat Toomey, Orrin Hatch and Susan Collins. Some in the group have already laid out their demands for a tax bill. Here’s a closer look at those senators, whose actions will be crucial to getting a tax bill across the finish line:

Having announced he’ll retire after the 2018 election, the deficit hawk is free to chart his path. And the two-term Tennessee senator is setting down a marker, insisting he won’t support a tax bill that adds to the deficit. That could make meeting Trump’s promise for massive tax cuts difficult.

Bob Corker

Photographer: Zach Gibson/Bloomberg

Corker recently struck a deal with Toomey paving the way for budget legislation that would allow for huge tax cuts in theory, but Corker has said he wouldn’t allow them to balloon the deficit. “With realistic growth projections, it cannot produce a deficit,” Corker said Wednesday. “There is no way in hell I’m voting for it.” He estimated that some $4 trillion in revenue-raisers must be achieved in order to ease his concerns.

Corker also said he wants to “get down to lower corporate rates and get rid of all these crazy issues that exist in our tax code,” describing his opposition to raising the deficit as a “hard stand” in order to “make sure we stay fiscally sane.”

This week, the Arizona Republican outlined the same condition on tax legislation that twice proved pivotal in blocking Obamacare repeal efforts in the Senate: Regular order that allows for hearings, debate, amendments and bipartisan support. “We need to do it in a bipartisan fashion,” McCain said Tuesday of a tax overhaul. “I am committed, as I’ve said before, to a bipartisan approach, such as we’ve been doing in the Armed Services Committee for the last 53 years.”

John McCain

Photographer: Andrew Harrer/Bloomberg

That may be difficult to reconcile with Senate Republican leaders’ plans to use the fast-track procedure on taxes that they tried to use on health care. Will the so-called maverick be satisfied? Initial reaction among Democrats to the tax framework indicates firm opposition, but McCain was more positive, praising the multiple tax hearings that the Finance Committee has held and saying he looks forward to reviewing the proposal.

McCain, 81 and battling brain cancer, has a history of bucking his party on the issue of taxes. He was among the few Senate Republicans to vote against President George W. Bush’s two separate tax cuts in 2001 and 2003, although he later came to support making them permanent.

The Kentucky libertarian is never an easy vote to win over — he proved it during the health-care debate by staunchly opposing the Senate’s last opportunity to undo Obamacare before the Sept. 30 deadline, complaining that it didn’t go far enough. And now he’s staking out a far-reaching position on taxes, too, calling for a “large cut of at least 15 percent for every taxpayer” in an Aug. 30 op-ed.

Rand Paul

Photographer: Andrew Harrer/Bloomberg

Paul is also opposed to paying for a tax cut, describing the push for revenue neutrality as “a terrible idea” that simply shifts around the tax burden and fails to achieve “real tax cuts.” He called on his party to reject the principle of revenue neutrality, warning it will “result in those with the best lobbyists, lawyers and accountants being the winners, while most everyone else either gets nothing or largely loses out.” Senator Ted Cruz of Texas has also said a tax plan should include big cuts, pushing back on the idea of revenue neutral changes.

In 2015, ahead of his failed bid for the Republican presidential nomination, Paul rolled out a “flat tax” plan that would impose a 14.5 percent individual rate.

The second-term Pennsylvania senator is an outspoken cheerleader for tax cuts and has argued against the need to pay for such a plan, saying the overarching focus must be on economic growth and that a revenue-neutral plan would be “anemic.” He successfully pushed for an agreement on a budget vehicle that allows the tax cuts to add to the deficit. He’s also argued for changing the rules to extend the budget window for a temporary tax cut from 10 years to as many as 30 years. “I’d like to stretch that out as much as possible,” he said.

Pat Toomey

Photographer: Pete Marovich/Bloomberg

Toomey, along with fellow tax wonk Senator Rob Portman of Ohio, are seen as the Senate Finance Committee’s thought leaders on tax policy. One of Toomey’s big priorities: “Expensing capital might be the most pro-growth element of this exercise,” he told reporters Wednesday. “That’s really really important to me.” Another is to create an incentive to bring home the estimated $2.6 trillion in corporate profits sitting overseas and set up a territorial system where U.S. companies aren’t subject to “an extra layer of tax from overseas income.”

A tax overhaul may be the GOP’s last best chance to secure a major legislative victory in 2017, and the chairman of the tax-writing Finance Committee views it as a political imperative. “Very important,” Hatch said in an interview Wednesday. “We all feel the pressure to deliver on taxes.”

Orrin Hatch

Photographer: Andrew Harrer/Bloomberg

Hatch, who was a second-term senator during the last big tax-code rewrite in 1986, is keenly aware of the issue’s complexity. He declined to say if he believes the Senate will secure a majority to eliminate the state and local tax deduction, a major revenue-raiser targeted in the Trump-GOP framework. “I’m not going to talk about specifics,” he said. “It’s a very complex bill to begin with. And we’re going to have to make some very tough decisions about what we keep and what we don’t.”

Hatch’s panel has a 14-12 split between Republicans and Democrats, which means he can afford to lose no more than one Republican if Democrats decline to play ball. He said he hopes to win Democratic support, but conceded that it “would be unique” in this political environment. “If they want to play politics with it, it’s another matter.”

Arguably the Senate’s most moderate Republican, the always-meticulous Collins drove a hard bargain on health care that helped torpedo the push. While she has said little about the upcoming tax debate, nobody is taking her vote for granted.

Susan Collins

Photographer: Andrew Harrer/Bloomberg

The Maine senator has voted for numerous tax cuts in the past, including the Bush tax cuts of 2001 and 2003. But she has also taken some heterodox positions that could be a factor: In 2015, she was the only GOP senator of 55 who broke ranks and voted against a budget measure calling for repeal of the estate tax, which the Trump-GOP framework seeks to do. In 2008, she voted for a measure to raise the top tax rate for people earning more than $1 million.

For now, Collins isn’t revealing any of her thoughts on the framework.